How can I create a normal map that points 90 degrees upward from a flat surface?
Normal maps are not necessarily for adding detail but to change the way light falls on the surface. Here is a quick example of changed normal map without changing the detail using the Normal node:



Here is an alternative with the normal map generated dynamically by calculating the direction between the object and the light (Can I refer to another object&#39;s properties from a Cycles node shader? Drivers? If so, how?):




Watch this excellent tutorial by 
Zacharias Reinhardt
https://www.youtube.com/watch?v=BtUS8QTsVZA&amp;index=4&amp;list=PLGodxM8aVtUfH9_xv4OdMM4xvBGZpggoA

in a nutshell, you have to


create your full object with material first
render it in ortho on a square plane
generate normal map and color texture for this square projection
use the normal and color textures as material for your "cross billboard"


To get a proper shadow is more complicated but can be achieved as per the great trick of part 2 of the video by Zacharias Reinhardt!